The movie opens with a “Pajama Party” (very few pajamas... robes, lingerie, and underwear) at the Theta Pi sorority house in the middle of hazing. The camera wanders through the party, and we meet the housemother Mrs. Crenshaw (Carrie Fisher) and five of the senior class sisters one by one: Jessica (Leah Pipes) is blond, bitchy and either the president of the sorority or self-appointed queen. Ellie (famous daughter Rumer Willis, looking exactly like she did in The House Bunny but without the orthopedic brace) is intelligent/pre-med, has glasses, and is the obligatory awkward less-sexy sister. Claire (Real World alumna Jamie Chung) who is sexy and Asian and looks like the love child of Lucy Liu and Tila Tequila. “Chugs” (Margo Harshman), who drinks a lot and makes nasty jokes, is the epitome of the drunken-slut sorority stereotype. And finally Cassidy (Briana Evigan) who is pretty, responsible, and thus our main character.

The girls gather in Jessica’s room for shots of tequila to celebrate their senior year and after toasting/roasting one another one-by-one (see above descriptions), sit on Jessica’s bed while she shows them “a future YouTube classic.”

On her laptop, we see a live feed of the room of Megan (The Hills’ Audrina Partridge) who is in bed with Chugs’ brother Garrett (Matt O’Leary). One girl comments that she looks comatose, and Jessica and Chugs reveal that they gave Garrett roofies. The others are in disbelief but eventually shrug it off since Megan and Garrett are an on-again off-again couple. Megan starts foaming at the mouth, and Garrett starts screaming, which causes the five to run over to the room.

Megan isn’t moving. Garrett and Ellie are both freaking out, and when Jessica asks him what the hell he did; he says he only gave her the pills. They tell him one would have been enough. Jessica gives him her keys and tells him to get her car so they can take Megan to the hospital. While he leaves, Jessica performs CPR. Once he’s gone, Megan pushes her off and asks if she’s going to use tongue or not. The girls who aren’t in on the prank realize that this is revenge for Garrett cheating on Megan.

The prank isn’t over though, so the others agree to play along. They pile into Jessica’s SUV, but Claire drives. Before they get to the hospital, they tell Garrett that Megan isn’t breathing and is definitely dead. Claire says in her hurry and panic she took a wrong turn. Jessica tells her to keep driving while Garrett wonders where they are going.

They end up at a lakeside, abandoned mine. They lay Megan down on Cass’s jacket. Garrett says they need to call the hospital or the police or anyone. He walks away to make a call, and Megan complains that with her eyes closed she can’t see anything [i.e. Garrett’s reactions and the others snickering]. Jessica tells her to shut up, so Megan lays her phone next to her and starts taking a video.

Garrett returns and says he has no signal. Jessica gives a speech about how they need to get rid of the body. Garrett says he’ll take responsibility if they turn themselves in, but Jessica says they’d all be accessories to murder. She asks whether they should try sinking Megan into the lake with weights or chop her up into pieces. Garrett is still completely freaked out, but the other girls nervously play along. Jessica tells everyone to scatter and look for sharp rocks to dismember the body with. As everyone walks away, she says they might have to puncture Megan’s lungs because if there’s air left in them, she’ll float. Jessica comments that she doesn’t know if the silicone in Megan’s breasts will cause her to float. From the cell phone camera’s point of view, we see Megan give a middle finger. Cass and Claire approach Jessica, and in whisper ask her how long this prank is supposed to take, AND GARRETT IMPALES MEGAN WITH A CROSS WRENCH. We see from the cell phone POV again. Megan starts coughing up blood, and everyone is in shock. Garrett, in tears, says he was puncturing her lungs and freaks out when he realizes she was still alive. Everyone tries to stop the bleeding, call 911, etc. Megan dies, and the cell phone signals still don’t work.

They have a new argument, similar to the first fake one but with everyone involved this time. Some people want to call the police, but their chapter would be disbanded, their lives would be over, and all their families, friends, jobs, relationships, and futures are in jeopardy. Jessica says Theta Pi sisters stick together. Cass counters with their sorority tenets, like Trust, Honor, and Respect. Jessica tells her she forgot the last two: Secrecy and Solidarity. Cass tells Jessica she’s only thinking of herself, but Jessica says she’s thinking of her boyfriend Kyle and his father the senator. She tells Cass she should think of her boyfriend Andy too. Ellie wants to fess up too until she remembers she, being the non-wealthy one, can’t afford to lose her scholarship, let alone criminal lawyers. Jessica eventually sells it to the others with the idea that, if she were Megan, she wouldn’t want the lives of the sorority sisters she loves ruined by this. Cass walks away, and says she’ll walk until she can find a signal. She’s calling the police.

Garrett is having a breakdown off-screen, so the remaining girls dump Megan’s body and cell phone down the mineshaft, followed by the cross wrench. Claire worries that this is wrong and that Cass will tell, but Jessica reassures her that Cass will return because she can’t walk all the way back. Eventually she does and can’t believe they’ve dumped the body. Jessica says they were only helping her, and that since they’re such good sisters they won’t tell anyone that Cass killed Megan. The others, including Ellie eventually, follow her lead. Cass says she can’t believe this. Jessica says they even threw down Cassie’s bloody jacket for her, the only other evidence. Defeated, Cass gives in. The girls wash off in the lake and return to the party to avoid suspicion, making a pact that they last saw Megan at the party and know nothing about disappearance.

Months Later...
It’s graduation day, and Cass’s boyfriend Andy (Julian Morris) is the valedictorian. He ends his speech with the idea that people judge you by the company you keep, and that he is proud to be in the company of his classmates. Everyone throws their hats, Chugs pops a bottle of champagne, and Cass is shown standing apart. Jessica waves, but Cass ignores her.

Andy and Cass are shown in bed the next morning. Andy, thumbing a Theta Pi invitation, asks what happened between Cass and the others since they used to be so close. She doesn’t want to talk about it and says they’re only going to the goodbye luncheon because she’s part of a legacy family  Her mom was in the sorority. Andy says after this she never has to see any of the girls ever again and that tomorrow morning they’ll be hours away at his parent’s vacation house and can just relax. Their plan is to drive away after the luncheon. The couple obviously loves each other very much, and Cass throughout the movie uses a GPS application on their cell phones to see where Andy is.

Cass, her parents, and Andy attend the luncheon at the sorority house and are greeted by Jessica, who says it’s been ages since they’ve seen each other. Jessica gives a goodbye speech, points out scaffolding for the renovations to the east wing, and offers a toast in honor of their missing sister. The other four think they see Megan approaching through the crowd, alive and beautiful, in a dress. Ellie screams and faints.

In the kitchen moments later, Jessica is yelling at everyone  The girl was Megan’s (biological) sister. Jessica tells everyone to keep it together because it’s their last night of college, for tradition they’re expected to throw this last party, and then they can all just move on. Megan’s sister Maggie (socialite Caroline D’Amore) comes in to apologize and says she’s going to pledge Theta Pi when she is a freshman in the fall. This is too much for Ellie, who leaves. Maggie asks if she can come to the party tonight. Jessica says that would be against rush rules, but Maggie can “Facebook her,” and she’ll personally ensure she gets a bid next year. The girls think Maggie is creepy, which she is, but Cass says given the situation she has every right to be.

Once Maggie is gone, one of the girls gets a photo via text message. It’s a gloved hand holding a bloody cross wrench. Everyone is scared but agree it’s Garrett playing a sick joke because he never returned to normal after the accident. Chugs says she’ll deal with her brother. Kyle and Mickey, Jessica and Claire’s boyfriends respectively, interrupt by barging into the kitchen and asking why they aren’t helping prepare the party. Chugs takes this as her cue to leave  she has an appointment with her psychiatrist  and Mickey, who is a douche, encourages her to blow the shrink for more extra Oxycontin. She says unfortunately he’s more of an ass-man and leaves.

Next, Mrs. Krenshaw is giving a speech telling the girls how special they are, how much she’ll miss them, don’t trash the house too much, etc, because it’s tradition that the house mother is absent the last night of the year. She gives them all bracelets made of 22 links (one for each graduating sister). Cass puts her on but most of our other main bitches toss them away after Krenshaw leaves. Jessica wouldn’t dare wear something “silver-plated?!” Cass meets up with Andy and asks if they can stick around for the party since she agreed to help Ellie set up. They agree to sneak out within the first thirty minutes.

We see Jessica meeting her boyfriend Kyle’s dad for happy hour or dinner or something. Kyle is nowhere to be found, so before he shows up his dad gives his possibly-future-daughter-in-law a speech about how he doesn’t like surprises and always reads the last page of a book first. He may get a nod for Vice President, so he doesn’t want her to bring any sort of trouble to his family. She assures him she would never.

At her shrink’s place, Chugs helps herself to a bottle of liquor from a shelf and then finds Dr. Rosenberg handcuffed to his bed. He says his last appointment left him like this, and she agrees to have sex with him for his latest sample prescription drugs. She goes to freshen up, and he hears strange noises and sees a Shadowy Figure approaching. He succeeds in unlocking the handcuffs but a flying cross wrench stabs him in the face. After trying to raid the medicine cabinet, Chugs says “Bottoms up, slut” to the bathroom mirror and downs more liquor before returning to the bedroom. The doc’s body is missing, so she walks around the house saying she doesn’t have time for “chase me, rape me” before she settles on his therapy couch. She brings the bottle to her lips again at a 180-degree angle and The Killer slams it into her mouth and throat, wedging it there. As she literally (and ironically) chugs for her life, he slits Chugs’ throat, filling the bottle with blood.
Back at Theta Pi, preparations for the party are in full swing. Claire convinces Mickey to have sex with her in the hot tub, despite all the people around them, for one last time. Afterwards, when he starts hitting on girls, she sends him home to get ready for the party while she takes a shower. Ellie is visibly shaken and Cass tells Jessica she’s worried about her. Jessica says since they’ve graduated, Ellie’s wellbeing (and thus Jessica’s term papers) isn’t her concern anymore before heading off to take a shower.

Claire finishes her shower and is surprised when someone opens the curtain. It’s a younger sister who didn’t realize she was in there. (It’s a big group shower room.) Jessica comes up behind her and scolds her for trying to sneak into the Senior Showers. As punishment, she makes the girl leave her towel and go back to her room naked. She implies Jessica is jealous of her “perfect breasts” and does it. In another shower, we see another non-senior girl praying she isn’t caught. Claire and Jessica have a heated conversation, which ends with Claire angrily saying they killed Megan. Jessica tells Claire she should just yell at her outside so the entire house can hear. She leaves. Girl-In-Shower drops her shampoo in surprise at what she heard. Claire hears it but is jumpy enough to just leave as well. When Shower Girl tries to sneak out, The Killer, who we now see is in a Hooded Robe, stabs her through her chin.

Downstairs in the unlit and creepy basement, Ellie is looking for vodka. Kyle surprises/scares her, asking if she needs any help and she sends him on his way. Since the light doesn’t work, she uses her cell phone for light but instead of vodka in the back she finds Cass’s jacket, still bloody and now ripped in half. She screams. (She screams more than anyone in this movie.)

The girls have another sisterly meeting in Jessica’s room. Jessica insists it’s Garrett, who must have gone crazy with guilt. They try calling Chugs but reach her voicemail, which tells them to leave a message after the [BELCH]. Some want to cancel the party, but Jessica says it’s tradition and doing something that bizarre would only look suspicious. She says she’ll take care of Garrett herself if Chugs won’t. Ellie thinks Megan either didn’t die or came back from the dead and is taking her revenge, which is what they all deserve.

Claire and Cassidy finish getting ready together and Claire apologizes, admitting they made a mistake and saying she admires Cass for all the charity work and self-punishment she’s inflicted on herself. Cass says they’ve all just made a series of mistakes since the accident.

The party starts (involving foam, bubbles, loud music, and lots of booze) and we see several hooded robes  Every graduating fraternity or sorority member got one per tradition. One hooded figure unveils herself to be a naked dancing chick. Mickey keeps trying to grind on Claire while they dance, but she’s not in the mood and eventually shoves him off and throws a necklace (presumably a gift from him) at him before storming off.

Mickey finds Ellie sitting silently outside and tries to comfort/seduce her because Claire broke up with him and hit him. She whispers to never touch her again, or she’ll kill him. After she leaves, he tries to bribe two underclassmen into flashing him by offering entry wristbands. The Killer interrupts them. The girls leave, and before Mickey can pick a fight, The Killer disappears. Mickey sees Ellie watching from a window in the east wing and asks her not to tell Claire. She ignores him and walks out of sight. He runs up there to plead his case but finds The Killer instead, who breaks his ankle. Mickey tries to limp away and escape through an open vent, but his throat is slashed through the wall. After The Killer exits, we see Ellie come on-screen from the opposite direction. We’re not sure how much she saw, but she clearly isn’t The Killer, and she sees the body. She screams and runs downstairs.

Jessica gathers her, Claire, and Cassidy in the kitchen for another session. Ellie is hysterical and hyperventilating, so since she can’t even say a word or syllable, Jessica makes her just show them what’s wrong. Jessica and Cass enter the room while Claire waits with Ellie in the hall. Mickey’s body is still there, and so is the cross wrench. The pair have trouble believing it’s the same one but note that it looks “pimped out.” Elaborate sharp weapons have been added to each of the four limbs. Jessica is convinced it’s Garrett, who’s gone from crazy to murderous. They receive another message on their cells. It’s from Megan’s phone, and it’s the video she took the night she died. No one else knew it existed, but all their voices and parts of them are visible in it. A text is attached that tells them to meet her where she died or in twenty minutes the video is going to the police.

Jessica tells Cass they need to remain calm, get the other two, and go to the mineshaft without letting Ellie see the latest text. Another scream from outside informs them Ellie has seen it. As they leave the room, Jessica mentions to Claire that Mickey happens to be dead. Claire is either shocked or under whelmed, it’s hard to tell.

Downstairs Jessica tells Kyle she has to take care of something. Kyle seems suspicious and worried (more for himself and his dad) and yells after her to take care of whatever it is. Meanwhile, Cassidy tells Andy to go ahead to his parents’ place without her. He objects, but she tells him if he loves her, he’ll do this favor for her. She tells him she loves him and will explain everything, absolutely everything, in the morning when she meets him there. She kisses him passionately until impatient car honks tell her it’s time to go.

Jessica, flooring it, almost hits two parked cars with her SUV on the way out and nearly runs over a girl standing in the road. She gets out to yell, and the girl is Maggie, Megan’s sister, who is creepier than ever. She says she’s going to the party because Jessica’s rush rules are fake. She also says Megan always hated Jessica and Megan knew that Jessica was the one Garrett slept with. Jessica says she doesn’t have time for this and will deal with her later. Maggie tells her she should deal with her hair first. As the girls drive off, Ellie remarks that Maggie would make a great Theta Pi.

Before reaching the mineshaft, they check the trunk of the SUV for any flashlights or makeshift weapons. The only useful object they find is flare gun. When they get there, Garrett is waiting for them. His hands are bloody, and he’s holding the other half of the jacket. Jessica gets back in her car, and Claire points the flare gun at him and tells him to stay back. He is hysterical, but he says the blood is only his own. He starts screaming at them and approaches with a sharp, shiny object. Before he can get to them, Jessica runs him over and kills him. When they check the body, they find out the blood is because he slit his wrists with a broken shard of mirror.

They debate whether he killed Mickey or not. Jessica stands by her beliefs, but Ellie is still convinced it’s Megan’s revenge. They decide to check the mineshaft. They lower Cassidy down with an old chain while Jessica lights the shaft with Garrett’s mirror shard. The chain snaps and Cass falls. Ellie screams, again, into Jessica’s ear causing her to drop the shard, which barely misses Cass, who isn’t injured. She uses her phone for light, but the body is gone. On the cave wall in blood are the words “THETA PI MUST DIE.”

When they park Jessica’s car at the house, the party is abandoned, but the music is still on, the outside is covered with trash or foam, and there’s an incessant clogged noise. Ellie thinks everyone is dead, but Jessica counters that corpses don’t drive their cars away. One sister on her way out tells them everyone moved on to some frat’s after-party and that the noise is from the hot tub, which his broken.

Jessica says she’s going to find Matt, and then they’re going to figure out what to do next. Cassidy and Ellie enter with her, but Claire is irritated by the hot tub noise, so she goes to turn it off. The buttons aren’t working, so she traces the power cord through the neck-high foam. Inside, Jessica is equally annoyed and is kicking out stragglers and unplugging the speakers. Ellie is quiet. Cass checks her phone and sees that Andy is out of state already. The peaceful silence is interrupted by new text messages, this time from Chugs’ phone: “Jessica, Ellie, Cassidy, I’m not coming to the party. Because I’m dead.”

Cass and Ellie gasp, but Jessica immediately asks: “Why isn’t it addressed to-“ Claire bangs on the French doors, screaming. As Cassidy starts undoing the many locks, she notices a loop of extension cord around Claire’s ankle. It tightens and she gets pulled back in the foam by The Killer. Cass runs out with a cane, and the flare gun goes off. She bats away foam, heading towards the light, and finds Claire dead with a flare in her mouth burning her face away.

Inside, the remaining three girls look for Kyle and find someone under the covers in Jessica’s room. It’s Maggie, and she claims to have slept with Kyle, who bailed when he saw Jessica’s car approaching. Jessica and Maggie have a catfight, rolling into the hall, until Cass and Ellie pull them apart. They hear footsteps and Jessica grabs a fire ax, leaving the extinguisher. She swings it as someone turns the corner. It’s Krenshaw, who ducks and bats Jessica across the face with her shotgun. She is pissed off and says she knows what they did. Cassidy blurts about maybe-killing Megan, which throws Maggie into another hissy fit, but before she can attack Jessica again Krenshaw fires a shell into the ceiling. She tells them to go to Jessica’s room and call the police. The three sisters obey, but Maggie runs off, excited by the prospect her sister might be alive. Once in the room, they realize they’ve all lost their phones at some point in the night’s antics, but the sounds of Krenshaw’s shotgun blasts comfort them.

Downstairs, Krenshaw is searching room by room and encounters The Killer in the kitchen. He throws the cross wrench, and she fires. They both miss. While she reloads, he slams into her, impaling her on the cross wrench lodged behind her. In a nearby room, Maggie and The Killer meet up. She asks if it’s Megan but is answered with a Molotov cocktail, which engulfs the room in flames.

Upstairs, Jessica and Cass decide to check Mickey’s body for a cell phone to call for help. Ellie hides in the closet (behind the hanging clothes, with her legs amusingly still visible). In the east wing, they find Kyle, dressed in a Greek robe with no shoes on. Cass takes this to mean he is The Killer, while Jessica takes this to mean he actually slept with Maggie. He has the fire ax and attacks them. He is apparently pissed that Jessica let all this happen, and, as his girlfriend, it’ll reflect badly on him and thus his dad. The girls run to an under renovation bathroom and find Megan’s decomposing corpse hanging in the shower. When they try to escape through the window, they realize the house is on fire. As Jessica checks the bathroom door, Kyle swings the ax, knocking her out. After chopping open the door, his next swing gets the ax caught in the doorpost so Cass runs out. He catches her, but right as he is about to kill her, Andy stabs him in the side of the head.

Cass kisses Andy and thanks him. She says she’s been checking his location, but he tells her he just put his phone on a Greyhound bus. He wanted to take care of her. She tells him he’s the best thing in her life... and then she sees the cross wrench in his pocket.

She can’t believe he’s The Killer, but he explains that he just wanted to take care of her. He can’t believe what the other girls have put her through and wanted them out of her life  and his life  forever. He is hurt she never told him the truth despite all the chances he gave her to. He repeats what he said at graduation, about the company you keep, and says Cass’s company was all bitches. Jessica has come to, and comes out of the bathroom. She says Andy is right, and they can just make up a new lie. He throws the cross wrench, right into her mouth, killing her and pinning her to wall. Cass says maybe they deserve to die, but all those innocent people. Andy says he has no intention of killing her. He wants to protect their future together. But he had to kill her former friends in case they went crazy or got drunk and told someone. And he had to kill everyone else they told.

Cass appears to agree with his logic, until he says all they have to do now is kill Ellie. She says Ellie is harmless, but he says if Ellie told him, she’ll eventually tell someone else. Cass lies and tells him Ellie is hiding in the basement. He goes off to find her, and she runs back to Jessica’s room. Ellie asks if it’s safe, and Cass says it’s not and they need to leave. In the hall, Andy lunges in between them. Cass tells Ellie to run. Andy hesitates for a moment but decides to chase after Cassidy. Eventually she knocks him out with the fire extinguisher.

She looks for Ellie but finds Maggie instead, cornered by the fire in another room. She tells Maggie to jump over, but Maggie says the fire is too wide and too high. Andy enters. He has the cross wrench again and ties it to a rope. He starts throwing it at the girls, reeling in each time he misses. The boards under Cass give in to the fire when she dodges, and she falls. Her life is saved by Krenshaw’s gift bracelet catching on a nail. Underneath her, the level below is engulfed in flames.

Andy stands above her, cutting her fingers with the cross wrench, saying how disappointed he is that she’d betray him after he’s even killed for her. He tells her this is the end for them and that valedictorian comes from the Latin vale dicere, which means to say... someone says, “Farewell asshole.” They both look up to see Ellie, who shoots Andy in the chest with Krenshaw’s shotgun. He falls back, breaking the floor and falling into the inferno below. Ellie helps Cass up and tells Maggie to wrap herself in the thick curtains on the windows behind her. Maggie does so and is able to run through the fire. Before they climb down the scaffolding to safety, Cassidy thanks Ellie for coming back to save her. To which Ellie replies: “What are sisters for?”

Months Later...
The Theta Pi house has been rebuilt, and Greek life is back in full swing as students frolic up and down sorority row. The new sisters, including Maggie, are singing a Theta Pi song while gardeners tend the lawn. The camera zooms in on one gardener with slit wrists carrying a sharp tool - it's Garrett.

*CUT TO THE CHASE*
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Megan and some of her sorority sisters pull a prank on her boyfriend Garrett, where they pretend he accidentally killed her. And then he accidentally kills her. Cassidy wants to go to the police, but Queen Bee Jessica convinces everyone not to and to extort Cass into agreeing by saying she did it.

Months later, a mysterious Killer kills everyone the girls told (a psychiatrist, two boyfriends, the house mother, and another sorority sister who takes a shower) and kills almost all the girls, ending with Jessica, who killed Garrett because she thought he was the Killer.

Only two survive. One girl is Cass, because The Killer is her otherwise-perfect boyfriend Andy, who wanted to free her of her bitch friends and make sure no one ever told and got her in trouble. The other girl is Ellie, who happens to be the only other girl who feels guilty, is smart, and not a total bitch. Oh, and the house is on fire.

But Andy wants to kill Ellie, so Cass saves her. Then Andy wants to kill Cass, so Ellie saves her by shotgun-ing Andy into a flaming pit. Ellie also saves Maggie, Megan’s creepy and slutty younger (biological) sister who slept with Jessica’s boyfriend.

Months later, Maggie is now a member of the sorority. One of the gardeners has a weapon and slit wrists; It's Garrett.